This invention relates generally to fasteners and other devices utilizing washers, and more particularly to such a device having a captured washer thereon and a method of producing such a device.
It is known to produce devices such as fasteners which include a washer captured on the device so that it may not be easily removed. One example of such a device includes a captive nut and washer assembly disclosed in U.S. Pat. No. 4,654,913 issued to Grube. A method of forming this device includes forming the nut and washer as a single blank. A frusto-conical groove is formed at an oblique angle to the axis of symmetry defining a separation between the nut and washer. An axial compression force is applied separating the washer from the nut along a frusto-conical fracture defined between the bottom of the groove and a concave or angled inner diameter of the washer. The orientation of the fractured surface causes the washer to be captured on the nut.
Another example is disclosed in U.S. Pat. No. 4,157,725 issued to Stanaitis. This type of fastener includes a hex head and an elongate shaft having a smooth outer surface. A washer is preassembled onto the smooth shaft after which threads are roll formed into the smooth surface. The formation of the threads increases the diameter of the shaft to the major diameter of the threads to capture the washer on the fastener.
A further example is disclosed in U.S. Pat. No. 5,308,285 issued to Malen. In this type of fastener, the shaft of the fastener includes a raised portion having an outer diameter which is slightly greater than an inside diameter of an opening of a washer. The washer is forced over the raised portion wherein it elastically deforms. Once thc washer passes over the raised portion, its inside diameter returns to a diameter slightly smaller than the raised portion to capture the washer on the shaft of the fastener.
One problem with the fastener of the Grube reference is that the fractured surface is formed on an angle. In order to prevent the washer from being removed from the nut over the frusto-conical surface of the fracture, the length of the fracture must be significantly large. If this is not so, the washer may be forced over the frusto-conical surface with relative ease. The large or longer the surface fracture, the more difficult it will be to break the washer from the nut which also may result in an improper fracture.
One problem with the Stanaitis fastener is that it requires additional steps in forming the fastener. The washer and the bolt are formed separately and then assembled to one another prior to the additional steps of forming the threads on the fastener. Additionally, the washer of this type of fastener may sometimes be removed from the fastener simply by rotating it in order to utilize the threads to drive the washer toward the free end of the fastener. One other problem with this type of fastener is that again, the difference in the inside diameter of the washer opening and the outside diameter of the threads is relatively small which may permit the washer to be forced from the fastener over the threads.
One problem with the Malen et al. fastener is that since the washer is elastically forced over the raised bump in order to place the washer on the fastener, it may be forced back over the raised bump and removed from the fastener at least as easily. Additionally, with this type of fastener the washer and the fastener are formed separately as with the Stanaitis type fastener.